Oscillation suppressor for electron guns



July 11, 1961 R. A. DEHN OSCILLATION SUPPRESSOR FOR ELECTRON GUNS FiledNov. 5, 1959 0 6 llll /m enfor: RUdO/ph A. Dehn, by /w d &M

United States Patent i ce Patented July 11, 1961 2,992,3`47 OSCILLATIONSUPPRESSOR FOR EIJECTRON GUNS Rudolph A. Dehn, Schenectady, N.Y.,assignor to General Electric Company, a corporation of New York FiledNov. '3, 1959, Ser. No. %50,701 8 Claims. (Cl. 313-82) This inventionrelates to an electron gun of the type useful for producing an electronbeam in an electron tube apparatus and more particularly to the focusingdevic'e of such a gun.

In certain electron tubes of more recent development, high energyelectron beams are required for proper tube operation and to achievesuch high energy, the electrons in the beam are accelerated from thecathode, from which they briginate by emission, by high potentialsapplied to electrodes spaced from the cathode. In controlling the beam,it is frequently advantageous to provide a focusin'g cup or electrodeabout the cathode and having an open side in the direction of theaccelerating electrodes. The efiect of such electrodes is to confineofocus the electrons into a beam of relatively small radial dimenson.

In the -apparatus of the type described, the cup is contained in asealed conductive housing which is spaced from the cup. It frequentlyoccurs that the transit time of the high energy electrons in travellingfrom the cathode across a portion of the region within the housing is sointerrelated with the size and Volume within the housing, that it actsas a resonator so as to excite and establish modes of high frequency,spurious oscillations within the housing and including the region of thecathode and focusing cup. Such spurious oscillations are undesirable inthat they interfere with the direct electric field in'the space betweenthe cathode and accelerating electrode and thus, interfere withformation of the proper electron beam. In the absence of such spuriousoscilla t-ons, proper beam focusing may be readily achieved. Focusingcups made entirely of metal have no efiect in suppressing or dissipatingthe oscillations of the generated field in that such cups present a toolow resistauce to fields of the spurious oscillations. I

It is accordingly a principal object of my invention to facilitate highenergy electron beam focusing without production of any spuriousoscillations in the region of the beam focusing electrode.

It is another object of my invention to prevent the initiation orproduction of spurious high frequency electromagnetic energy waves inthe region about an electron gun. i

While the specification concludes with claims particularly pointing outand distinctly claiming the subject matter which I regard as myinvention, the invention may be better understood from the followingdescription taken in connection with the accompanying drawings in which:

FIG. 1 is a cross sectional elevation of an electron gun according to myinvention, and

FIG. 2 is an enlarged, partial View showingrin detail the film appliedto portions of the focusing cup. In accordance with my invention, theproduction and efiect of spurious oscillations in the region of anelectron gun are prevented or avoided by the provsion of a thin coatingof conductive material of predetermined resistivity over the surfaceportions of an insulating, focusing cup which are in regions tending toproduce a relatively high intensity, high frequency, electromagneticfield. Such a film coating is preferably of a refractory metal and is ofa resistauce such as to generally match the wave impedance of theoscillations tending to be produced whereby currents tending to beestablished in the film due to the efiect of high frequency oscillationsare most eiiectively loaded in the production of heat in accordance withthe product of the resistauce of the path of the cunrent and the squareof the value of the current. Accordingly, the oscillations themselvesare prevented from occurring. i

Referring now more particularly to the drawings for a more detaileddescription of my invention, 10 represents generally the entire electrongun structure which embodies my invention which includes a housingcomprising -a hollow, cylindrical, ceramc member 12 sealingly closed atone end by a wall 14, preferably of a suitable metal, and being inabutment with an end of a conductive cylindrical housing portion 16 atthe other end.

To complete the housing, an apertured anode 18 is in abutment with theother end of metallic housing 16. The anode section 18 is provided withan aperture 20 for accommodating the flow of electrons in the beamproduced by the electron gun. Other apparatus of the electron tubebeyond the anode 18, for which the gun 10 provides the electron beam, isnot shown, iasmuch as the same forms no essential part of the presentinvention.

Disposed within the housing '16 is a focusing cup 22 made of a suitablerefractory ceramic and supported by a tubular member 24, which in turnis rigidly Secured in an aperture of end wall '14 as by suitablesoldering or weldng. The interier end of tubular member 24 is receivedin an eniarged portion 26 of an opening 28 in the base of the cup 22 andmay be bonded thereto by any suitable means. The exterior end of tube 24is sealed by a cap 30 which is flanged at 32 to abut a complementaryfiange` 34 at the end of the tube 24 and is further provided With asupporting insulator 36 therewithin. -For conducting electrical energywithin the gun, insulator 36 is apertured to accommodate a conductiverod 38 extending along the interior of the tube. For producing a beam ofelectrons, cup 22 is opened at 40 and is provided with a recess 42 foraccommodating a cathode -44 having an outwardly concave surface. Thecathode is coated with a suitable emission enhancing materal 46 and issupported in the bottom of the recess 42 by a cylindrical foil support48 extending from the periphery of the cathode to the bottom of` therecess 42 and Secured at each end. For raising the temperature ofcathode 44 for copious thermionic emission, a heater 50 is mounted inclose proximity to the side thereof remote from the concavej, coatedside and electrical power is supplied to the heater 50 through the rod38 at the end of which is an electrical wire 52 extendng from the rodend, to one end of the heater. A suitable insulating grommet 54 disposedin -a conductive supporting member 56 spanning the aperture 28,accommodates the wire 52. The other end of heater 50 is connected to thesupport ing member 56 which in turn is in conductive contact with tube24 through a wire 58 attached to the support ing member 56 and to thetube 24. j

In Operating the electron gun, a high potential diiierence isestablished between the housing 16 and the anode 18 on the one hand andthe cup 22 and parts conductively connected therewith on the other hand.To minimize adverse eflects of such high potentials at certainlocations, a corona shield is formed at the end of housing 16 as shownat 60 and a complementary corona shield is formed at 62 by a cylindricalmember 64 in contact with the end 14 and extending therefrom toward thehousing 16. These corona shields provide a smoothly curved surfaceexposed to the regions of high electric field, thus minimizing theconcentration of such fields at sharply pointed conductive edges withinthe gun.

The region within the housing 16 is of a high electric field becausehousing 16 and the cup 22 are, respectively, at aceclerating anode andground potentials. Electrons traveling between cathode 44 and anode 18may be of such a velocity as to have a transit time interrelated withthe dimensions of housing 16 so as to eXcite modes of high frequencyoscillations as in a cavity resonator.

In accordance with a feature of my invention, to prevent or minimizesuch oscillations, the cup 22 is made of an insulating ceramic materialsuch as alumina, beryllia, forsterite or steatite, for example, and iscoated with a conductive material along portions of high electric fieldintensity to provide an eifective load to such high frequencyoscillations to completely suppress the high frequency electromagneticfield tending to be generated in the region. To this end, as shown moreclearly in FIG. 2 of the drawing, the cup may be coated by a conductivematerial along portions 64, 66, 68, and 70; The material so utilized maybe 'any material which in a thin coating is conductive but presents aresistance which effectively matches the wave impedance of theoscillations tending to be created and to dissipate the energy of suchfields. This material further possesses required properties ofcompatibilty with other tube components as to matters such as low vaporpressure and has adequate potential breakdown resistance. Materialsproducing particularly good results and considered preferable aremolybdenum, tantalum and tungsten or any other suitable high refractorymetal. The use of these metals facilitates operation of the gun athigher potentials because of higher breakdown potentials thereof andthey are also possessed of qualities such as low vapor pressure athigher tube Operating temperatures.

In accordance with my invention, film thicknesses for optimum loading,may be varied over a rather wide range in accordance with the demands ofthe particular tube structure and the particular modes and frequenciesof oscillations tending to be produced therein. Film resistivities from100 ohms per square to 1,000 ohms per square are contemplated as beingoptimum values in different situations and under some circumstancesresistvities from to 10,000 ohms per square may be utilized effectively.

As pointed out hereinabove, for most efiective prevention of spuriousoscillations the film would be of a thickness to most effectively matchthe wave impedance of the spurious oscillations tending to be generated.However, in a practical embodment of the invention, certain portions ofthe film which are in portions of higher applied direct electric field,are preferably made somewhat thicker than is optimum for spuriousoscillation revention. Such thicker film portions are better able towithstand the destructive effects of occasional arcing that occurs atsuch regions by reason of the higher applied fields without beingsufliciently mismatched as to materially lose their effectiveness inpreventing spurious oscillations.

In the embodment of my invention shown in the drawings, along the filmportion 64, extending from the bottom of the cup interier to the curvedportion 66 and the film along portion 68, extendng along the exterior ofthe cup from portion 66 to curved portion 70, the applied directelectric field intensity is not as great as that along portions 66 and70. Thus, the coatings of film along the portions 66 and 70 arepreferably of lower resistivity than the coatings along portions 64 and68, so as to reduce high voltage `arcing possibilities. The relativelyhigher resistivity of the film along portions 64 and 68 achieves dampingof the high frequency oscillations.

While the present invention has been described by reference toparticular embodiments thereof, it will be understood that numerousmodifications may be made by those skilled in the art without actuallydeparting from the in vention. I, therefore, aim in the appended claimsto cover `all such equivalent variations as come within the true spiritand scope of the foregoing disclosure.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. An electron gun for producing a focused beam of electrons comprisinga cathode, `a ceramic focusing cup enclosing the cathode and anapertured anode, and means for preventing spurious oscillations in saidgun structure when said anode is operated at a high direct potentialwith respect to the remaining electrodes comprsng a coating of highrefractory metal along the portions of sad focusing electrode in regionsof high field intensity.

2. An electron gun for producing a focused beam of electrons comprisingan apertured anode, a focusing electrode and a cathode disposed withinsaid foousing electrode, said focusing electrode being a ceramicinsulating member having a coating of refractory metal along portionsthereof in regions of high field intensity.

3. A focusing cup for an electron gun comprising a hollow ceramic memberopen at one end for receiving a thermionically emissive cathode andportions of said member being coated with a high refractory conductivematerial.

4. An electron gun comprsing a hollow, conductive member and `aninsulative focusing cup within said member and spaced therefrom andportions of said cup being coated with a thin film of conductivematerial.

5. An electron gun comprising a hollow, conductive member having ananode at one end and an insulative focusng cup within said member spacedtherefrom, an electron emissive cathode within said cup for producing abeam of electrons in response to a potential applied to said anodepositive with respect to said cathode, means for preventing theproduction of spurious oscillations within said member by electronstraversing the space between said cathode and anode and including a thincoating of refractory, conductive material along portions of said cuptending to be in electromagnetic fields of high intensity.

6. An electron gun comprising a hollow, conductive member having ananode at one end and an insulative focusing cup within said memberspaced therefrom, an electron emissive cathode within said cup forproducing a beam of electrons in response to a potential applied to saidanode positive with respect to said cathode, means for preventing theproduction of spurious oscillations within said member by electronstraversing the space between said cathode and anode and including a thincoating of refractory, conductive material along portions of said cuptending to be in electromagnetc fields of high intensity, the resistanceof said film being matched to the wave impedance of spuriousoscillations tending to be produced in said member.

7. An electron gun for producing a focused beam of electrons comprisinga ceramc focusing cup enclosing a thermionically emissive cathode, ananode spaced from said cup, said cup being contained in a conductiveenclosure and means for suppressing spurious oscillations in saidenclosure when said anode and enclosure are at a high direct potentialwith respect to said cup comprising a coating of high refractoryconductive material along the portions of said focusing cup of highfield intensity, the portions of said cup in fields of greater intensityhaving coatings of lower resistivity than portions thereof in the lo pte ti l fields.

5 6 8. An elect'on gun comprising a hollow, conductve intensity, theresistance of said film being between 10 and member having an anode atone end and an insulatve 10,000 ohms per square to prevent thegeneration of focusing cup within said member spaced therefrom, anspun'ous osc'uations in said member electron emissive cathode withinsaid cup for producing a beam of electrons in response to a potentialapplied 5 References Cited in the file of this patent to said anodepositive with respect to said cathode, means for preventing theproduction of spurious oscillations UNITED STATES PATENTS within saidmember by electrons traversing the space be- 1 273523 Langmuir J l 231913 tween said cathode and aode and including a thin coat- 3 44 Birdsan5 1958 ing of refractory, conductive material along portons of 10%916,659 Sege 8, 1959 said cup tending to be in electromagnetic fieldsof high

